Fuse



E. H. YON KERS Nov. 9, 1948.

FUSE

Filed May 3, 1944 IN V EN TOR. E'cz/ardlnker BYWUM'; rWuau m 1os 102101 1 MELTING TIME IN SECONDS.

- Patented Nov. 9, 1948 FUSE Edward H. Yonkers, Chicago, Ill., assignox to Joslyn Mfg. and Supply Co., Chicago, Ill., a corporation of Illinois Application May 3, 1944, Serial No. 533,824

19 Claims. l

The present invention relates to electrical fuses and more particularly to an improved fuse link which is well adapted for use in providing current overload protection for a power distribution transformer of low KVA capacity and yet is itself protected against rupture when subjected to lightning surges. This application is a continuation in part of copending application Serial No. 514,030, filed December 13, 1943.

One of the problems involved in providing fuse protection for a power distribution transformer energized from a high voltage overhead distribution line is that of preventing the fuse from rupturing when subjected to a lightning surge that may cause high current flow through the fuse for only a few micro-seconds. This problem is particularly troublesome in the use of fuse links having fractional ampere ratings to protect transformers having low KVA ratings and equipped with primary windings adapted to operate at high voltages. In the usual commercial fuse link, no facilities whatever are provided to eliminate this problem. In the patent literature, however, this problem is referred to and various solutions are suggested. These suggestions all embrace the concept of connecting a surge gap in parallel with the fusible element of the fuse to by-pass lightning surge currents around the fusible element. The structures proposed for this purpose, however, either lack effectiveness in building up a surge gap breakdown voltage .or are wholly impractical from the standpoint of commercial production.

It is an object of the present invention, therefore, to provide a fuse of improved construction having exceedingly simple surge gap facilities for by-passing lightning surge currents around the fusible element thereof.

Accordingly to a further object of this invention, relatively immovable electrodes are provided in shunt with the fusible element of a fuse link to form a surge gap of fixed width which is not altered incident to rupture ofthe fusible element.

In accordance with another object of the invention, an impedance element is connected in series with the fusible element across the surge gap in order to enhance the build up of a break down voltage across the gap when the fuse is subjected to a lightning surge.

More generally, it is an object .of the present invention to provide a fuse link having a timecurrent fusing characteristic which approximates with tolerable accuracy the time-overload characteristic of the device it is utilized to protect,

and yet is of simple and rugged construction, is small in size, and is capable of being easily and economically manufactured in production quantitles.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawings, in which:

Fig. 1 is a side view partially in section illustrating the present improved fuse link when assembled within a fuse tube:

Fig. 2 is a side sectional view of a fuse link characterized by the features of the present invention, which view also schematically illustrates the manner of inserting the link in a transformer primary circuit;

Fig. 2A is a fragmentary side elevational view of the link shown in Fig. 2;

Fig. 3 is a graph illustratingthe time-current characteristics of fuse links of different ratings when constructed inthe manner shown in Fig 2 of the drawings; and

Fig. 4 is fragmentary side sectional view of a different structural embodiment of the invention.

Referring now to the drawings and more particularly to Fig. 1 thereof, the present improved fuse link is indicated generally at I Il as being supported within a hollow fuse tube which is formed of a suitable insulating material. At its upper end, the tube II is provided with a brass contact ferrule I2 which is suitably pinned to the upper wall portion of the tube by means of pins 9, and is provided with an inwardly extending annular portion I2a which overlies the upper end of the tube. The ferrule l2 is also provided with an enlarged rib portion l2b which may be appropriately positioned within the contact fingers of a fuse tube holding arm of the character conventionally embodied in cutout assemblies, with the sharp lower edge of the rib engaging the contact fingers. The upper portion l2c of the ferrule is provided with internal threads for receiving the threaded portion I5 of an assembly cap I4. This cap is provided at its upper end with a ring I5 which may be used to facilitate assembly of the connected parts Il, Ill, and I4 upon the supporting and connecting arms of a cutout assembly by means of a hot-stick or the like. At its upper end, the fuse link I0 is provided with an enlarged head I8 which functions as one terminal thereof and is clamped between the assembly cap I4 and the inwardly extending flange portion of the ferrule I2. A pigtail conductor I1 projecting from the lower end of the link I functions as the opposite terminal thereof. With this arrangement, contact of the ferrule i2 with the contact fingers of a supporting arm may be utilized to provide an electrical connection between one terminal of the fuse link and'one side of the supply line, and the circuit through the fuse link may be extended by means of the pigtail conductor I1 which is suitably anchored between the contact fingers of a second arm of the cutout assembly. More specifically and as best shown in Fig. 2 of the drawings, the fuse link Ill may be serially included in the circuit for energizing the primary Winding |9a of a transformer i9 from an open wire high voltage current supply circuit which includes the two bracketed conductors 20.

Briefly considered, the fuse link I comprises a metal tubular casing member 22 having, at its upper end, an outwardly extending flange 22a which is embraced by the inturned edges of the metal contact head I8. At its lower end, this tube is provided with an inturned portion 22h, the purpose of which is explained below. Disposed within the tube 22 and arranged in series circuit relationship between this tube and the pigtail conductor i1, are three fusible elements 23, 24 and 25. The two elements 24 and 25 are both formed of Nichrome wire or another metal wire or ribbon of the desired resistivity, and are both of the same cross sectional area, such that they possess substantially identical time-current fusing characteristics. The fusible element 24 is substantially straight throughout the major portion of its length 'and the lower end portion thereof is centrally disposed within the upper end portion of the stranded pigtail conductor il, the overlapping'portions of the two elements il and 24 being telescoped within a metal assembly sleeve 28. This sleeve is crimped adjacent the upper and lower ends thereof, as indicated at 28a and 28D, and the lower end of the fusible element 24 is brought out through the strands of the conductor l1 and bent over the lower edge of the sleeve 28 so that the portion 24h thereof overlaps the outer surface of the sleeve. After the three elements 23, 24 and ll have been assembled to occupy the relative positions illustrated in Fig. 2 of the drawings, the crimps 28a and 23h may beV formed around the upper and lower portions of the sleeve 28 for the purpose of providing a rigid mechanical connection between the three named parts. Thereafter, the lower end portion of the sleeve 28, the adjacent portion of the pigtail conductor il and the overlapping end portion 24o of the fusible element 24 may be soldered to provide a rigid connection therebetween.

The fusible element 25 may more properly be designated a combination impedance element and heating element in that it functions in conjunction with the improved surge gap facilities described below to prevent surge currents of large magnitude from traversing the fusible elements 23 and 24 and also acts to heat the fusible element 23 to a fusing temperature when the link is subjected to an overload current for a sustained time interval. In order to perform these two functions the element 25 is constructed in the form of a helical coil, and the upper end portion. 25a. thereof is electrically and mechanically connected to the inner side wall surface of the tube 22 by means of a body of high melting point solder 25e. The turns of the element 25 are spaced apart axially of the tube 22, and the spaced apart relationship between the turns is maintained by embedding the t -4 same in a body of dielectric refractory material`2l. This body is preferably formed of a refractory cement and serves several functions which are pointed out with particularity below. It may, for example, be formed of Portland cement or any ceramic material which is chemically inert, has high specific heat, and is possessed of good electrical insulating properties. In order to increase the inductance of the element 25, thereby to enhance the surge current blocking function thereof, particles 32a of magnetic material, such, for example, as iron powder or magnetite, may be dispersed throughout the body 21, but in no case should the density of the magnetic particles be such as to provide conductive paths capable of short-circuiting the turns of the element 25 or connecting any one of these turns with the tube 22. Among other functions, the body 2l serves rigidly to position or support the turns of the element 25 within the tube 22, and to this end entirely fills the upper portion of the tube. It also serves to support a tubular conductor 26 centrally of the tube 22, this element being utilized in the connection of the fusible element 24 with the lower vend portion. of the combination heating and inductance element 25. More specifically, the upper tubular portion of the connecting element 26 is projected well within the turns of the element 25 in spaced apart relationship therefrom, and is embedded in the body 2l. At its lower end this connecting element is provided with an outwardly e'ztending flange 26a which serves to seat the lower turn of the element 25 in a manner such that the tubular portion of the element 26 is substantially concentrically disposed within the turns of the element 25, This lower turn of the element 25 is electrically and mechanically connected to the anged portion 26a of the element 26 by means of a high melting point solder 25h, or the like. The upper end of the connecting element 26 is electrically and mechanically connected to the upper serpentine end 24a of the fusible element 24 through the fusible element 23, the latter element being in the form of a body of alloy solder having a melting point of approximately 365 F. It is to be noted that the fusible element 23, as thus formed within the tubular portion of the con.- necting element 26, is disposed well within the turns of the element so that heat generated by current conduction through the latter element may be transferred to the fusible element 23 through that portion of the refractory body 2'! which separates the fusible element 23 from the adjacent turns of the element Spring tension imposed upon the free lower end of the pigtail conductor il may be utilized to rapidly withdraw the end 24a of the fusible element 2t from the connecting element 26 when the named fusible element is heated to a melting temperature, and to widen a break in the fusible element 2li occasioned by heating this element to a fusing tem perature.

In order to maintain the turns of the element 25 out of contact with the metal casing 22, to maintain the element 2li out of contact with the inturned end 22h of the tubular casing 22 and to provide a support for the surge gap facilities described below, the entire assembly within the casing and a part of the sleeve 2t are surrounded by a tube 29 formed of Bakelite or other suit able insulating material. This tube fits snugly within the tubular member 22, and the lower end portion 29e thereof, which is of reduced external diameter, projects outwardly from the open lower end of the casing. It is counterbored from the sans upper end to provide a first portion 29h of large internal diameter for receiving the parts 25, 2i

and 2l, and a second portion 29o, 29e of smaller internal diameter for receiving the fusible element 2l. The ange 29a of the tubular conductor '26 is seated upon the step 29a between these two portions of the tube 29.

As indicated above, in order to prevent lightning surges from rupturing the fusible elements within the tube 29, a surge gap having relatively immovable electrodes is connected in shunt with the three series connected elements 24, 23 and 25. The casing 22 comprises one electrode of this gap. The second electrode comprises a metallic sleeve 3D which is xedly secured to the projecting portion 29e of the insulating tube 29 and is spaced from the open lower end of the casing 22. Thus an annular surge gap 32 of xed width is formed between the adjacent ends of the two metal parts 22 and 39. At its lower inturned end, the sleeve 39 is provided with separated resilient fingers SIb which extend longitudinally of and are tensioned against the sleeve 28 to connect the gap 3.2 in shunt with the series connected elements 23, 24 and 25. These fingers are separated longitudinally thereof by slits 3Ia to permit free flexing in directions radially of the sleeve 28.

In the assembly of the above described parts of the fuse link I0, the soldered connection between the upper end 24a of the fusible element 24 and the tubular portion of the connecting element 26 is first made through the fusible body 23, following which the heating element 25 is telescoped over ythe tubular portion of the connecting element 26 and soldered to the flange portion 26a thereof. Following these operations, the lower end of the fusible element 24 is inserted through the sleeve covered strands of the pigtail conductor Il, with the end 2lb thereof being brought out and bent over the edge of the sleeve 28. The next operations are those of crimping the sleeve 28 at the portions 28a and 29b thereof, and of providing a soldered connection between the lower ends of the sleeve 28, the end portion 24h of the element 24 and the portion of the pigtail conductor I'I which is adjacent the lower end of the sleeve 28. As a separate assembly operation, the insulating tube 29 is inserted into the tubular casing member 22 through the open upper end thereof until the external step 29d abuts the inturned lower edge portion 22h of the casing. The sleeve 30 may now be telescoped over the proj ecting portion 29e of the tube 29 until the transverse portion 39a thereof abuts the lower end of the tube, thus automatically fixing the width of the surge gap 32. A driving fit between the two parts 29 and 30 may be relied upon to maintain the sleeve 39 in its assembled position on the tube projection 29e. The fusible element assembly may now be positioned within the tube 22 by threading the pigtail conductor I1 through the insulating tube 29 and the resilient fingers 3Ib. The end 24h of the fusible element .24 may be pressed into the sleeve 28 in order to permit the lower end of this sleeve to pass through the restricted portion of the sleeve 30 and between the fingers 3Ib. Movement of the assembly through the tube 29 is obviously arrested when the flanged portion 26a of the tubular conductor 26 is moved into engagement with the internal step 29a of the tube. Incident to this operatiointhe conductive sleeve 28 is drawn between the fingers 3 Ib to spread these fingers apart and thus create a spring tensioned contact between the sleeve and upper cavity formed within the insulating tube 29 with the refractory cement or the mixture of refractory cement and magnetic particles. This may be accomplished by simply pouring the cement within the tube through the upper end thereof until all portions of the element 25 are embedded therein. During this pouring operation the connector 26 is also embedded within the cement, and the cement is prevented from entering the lower chamber within the tube 29 due to the sealing engagement of the connector flange 26a with the step 29a. After the Ibody of refractory cement has been allowed to set, or has been baked to enhance the setting operation, an appropriate die assembly operation may be utilized to attach the terminal head I8 to the upper end of the casing 22. Following the last mentioned operation the fuse link I0 is fully assembled and ready for use.

In considering .the mode of operation of the fuse link I9, it may be assumed that this link is to be serially included in the primary circuit of` the transformer I9 for the purpose of protecting this transformer against damage occasioned by current overloads. It may be assumed further that the transformer I9 is provided with a low voltage secondary load which under normal conditions approximates the full load capacity of the transformer I9, and that this secondary load includes motors and other devices which, durin'g the starting periods thereof, are capable of producing heavy transient currents of relatively short duration in both the primary and secondary windings of the transformer I9. In

this regard, it will be understood that due to the heat radiating capabilities of the transformer parts, the transformer I9 may be capable of withstanding an overload current of reasonable magnitude such, for example, as 200 .to 300 per cent for a relatively long time interval; whereas it can withstand current of the `order of 500 per cent of normal for only a short time interval. It will also be understood that the transformer is capable of being damaged by a sustained increase in the voltage appearing between the conductors 20 of the supply cincuit.

The fuse link I0 operates to protect the transformer against damage occasioned by overload currents caused Vbv any one of the mentioned factors. At the same time, the fuse link permits the transformer to be operated under sustained overload current conditions for a period less than that required for damage to the transformer, and will not rupture when subjected to the normal and non-injurious high currents which are produced incident to motor starting, or the like. In this regard it will be understood that since the three serially related elements 25, 23 and 24 of the link I0 are traversed by the current flowing through the primary winding ISa of the transformer I 9, they are all heated by current conduction and that the temperature of each element varies with changes in the magnitude of this current. The fusible element 23 is also element 23 throughthat portion of the refractory body 21 which is disposed between the tubular portion of the connecting` element 26 and the surrounding turns of the element 25. Under constant load current conditions and with a constant voltage between the conductors 20 of the current supply circuit, the amount of energy transferred to the fusible element 23 per unit time element interval remains substantially cons-tant. Accordingly, so long as the load current through the secondary winding i917 of the transformer does not substantially exceed the full load capacity of this transformer, the fusible element 23 is operated at a temperature well below that required to produce fusion thereof. When, however, the secondary load current of the transformer I9 gradually rises and is sustained for a period approaching that at which the transformer will be damaged, the temperature of the fusible element 23 is raised accordingly. Thus, as the load current increases, the current traversing the three elements 23, 24 and 25 is correspondingly increased so that more heat is produced in the fusible .element 23 by current conduction. Concurrently the amount of heat conducted to this fusible element from the fusible elements 24 and 25 is increased. Also concurrently, the amount of heat transferred from the turns of the element 25 to the fusible element 23 through the refractory body 2li is increased.

After a predetermined time interval, required for the accumulation of heat within the fusible element 23, this element is heated to its fusing temperature and melts. When this occurs, the fusible element 24, the sleeve 28, and the upper end of the pigtail conductor il, are quickly expelled from the lower ends of the tubes 22 and 29 under the influence of the spring means connected to the pigtail conductor, thereby rapidly to break the circuit for energizing the primary winding lga of the transformer i9. The sliding engagement of the fingers Sib with the sleeve 2b does not appreciably impede withdrawal of the named parts from the tube 29.

To consider the action of the fuse link lll further, it may be .pointed out that the refractory body 2l prevents the fusible element 23 from being heated to its melting temperature when surge currents are produced in the primary circuit -of the transformer lil as a result of motor starting or the like. short duration, being of .the order of only a few seconds. The resulting momentary increase in heat generation within the fusible element 23 is wholly insufficient to raise the temperature of the element to its melting point. Moreover, that portion of the refractory body 2l which is disposed between the tubular portion of the connecting element 2B and the turns of the heating element 25 dissipates a large portion of the heat resulting from the current surge through the element 25 away from the element 24. It also delays the transmission of the increased increment of heat produced by the element 25 to the adjacent walls of the connecting element 26 for an interval which will normally exceed the duration of the current transient. Accordingly, the increased increment of heat energy arriving at the surface of the connecting element 26 from the element 25 as a result of the momentary high current, effects an insufficient increase in the Such surge currents are of temperature of the fusible element 23 to cause this element to melt. In other words, the total l heat accumulated in the fusible element 23 as a result of the transient high current is insumcient to heat this element to its fusing temperature. Thus it -will be understood that the refractory body 21, or more exactly the thermal impedance of this body, protects the fusible element 23 against outages occasioned by transient currents of the character which frequently occur in the load pattern of any transformer secondary load. This is accomplished moreover, without increasing the -thermal capacity of the fusible element 23 to a point such that it will provide no protection for sustained overload currents.

The thermal impedance of the refractory body 21 also prevents the fusible element 23 from immediately rupturing when the transformer I9 is subjected to a high current, such, for example, as that which is produced when the secondary winding |927 of the transformer i9 is shortcircuited. In the absence of an additional protective element, therefore, the transformer I9 could easily be damaged by an overload current of this character during the period required t0 transfer sufficient heat from the heating element 25 to the fusible element 23 to cause the latter element to melt. The second fusible element 24 functions to guard the transformer I9 against damage when subjected to an overload current of this type, Thus, immediately the element 24 is subjected to a transient current of the particular character just referred to, a portion thereof lying between the upper end of the sleeve 28 and the lower end of the fusible element 23 is heated to a fusing temperature, permitting this element to rupture. As a result, the lower portion of the element 24 is expelled from the lower ends of the tubes 22 and under the influence of the pull-out spring. In this regard, it will be understood that the heat generated within thatportion of the fusible element 24 which is surrounded by the sleeve 28, is rapidly conducted away from the enclosed portion of the element 24 by the strands of the pigtail conductor il, whereby this portion of the element 24 is prevented from fusing when subjected to the described surge current of large magnitude.

As previously indicated, the elements 24 and 25 are so constructed that the inherent time-current characteristics thereof are substantially the same. It may be pointed out, however, that the body of refractory material 2l has substantial heat capacity;l whereas the air which surrounds the fusible element 24 has relatively little heat capacity. Accordingly, the heat generated by the fusible element 25 as a result of the heavy .surge current flow therethrough is absorbed by the body 2l at a relatively high rate, while the heat generated by the surge current ilow through the fusible clement 24 is only slowly absorbed by the air which surrounds this element. As a result, the temperature of the fusible element 24 rises much more rapidly than the temperature of the heating element 25, whereby fusing of the element 24 is insured before the element 25 is heated to its melting temperature. Thus, the fusible element 24 not only acts as a heavy current-short interval protective device for the transformer i9, but in addition, functions to protect the heating element 25 against damage when the fuse link i0 is subjected to heavy surge current overloads.

As previously indicated, the annular surge gap 32, as provided between the sleeve 30 and the lower end of the metal tube 22, serves to protect the fusible elements of the link I and more particularly the element 24 thereof, against rupture when voltage transients of short duration but high value and steep wave front appear between the conductors 20 of the high voltage current supply circuit. In this regard it will be understood that lightning or other disturbances may cause voltage transients to appear between the line conductors 20 which are of insuflicient duration to cause damage to the transformer |9 and yet may be of sufficient magnitude to cause the fast responding fusible element 24 of the ll'nk I0 to rupture. When a transient of this character appears between the line conductors 20, the voltage i. e. the current-impedance drop across the turns of the impedance element 25 instantly rises to a value sufficient to cause ionization of the shunt connected surge gap 32. If the wave front of the voltage ltransient is sufficiently steep, this breakdown of the surge gap 32 will occur before the fusible element 24 can be damaged. Once the gap 32 is ionized, the electrical resistance thereof immediately drops to an exceedingly low value such that the predominant portion of the surge current is by-passed around the fusible elements 23, 24 and 25 of the link i0. As the transient dies out, the voltage between the elements 22 and 30 decreases to a value insufficient to'sustain ionization of the gap 32, with the result that surge current flow across this gap is arrested. From the above explanation it will be understood that the impedance of the coiled heating element 25 delays the build-up of current flow through the fusible elements 23, 24, and 25 for an interval sufficient to insure breakdown of the surge gap 32 before the current traversing the fusible element 24 can reach a magnitude sufficient to cause this fusible element to rupture. Moreover, the par ticles 32a of magnetic material dispersed throughrut the refractory body 2l', in enhancing the electrical inductance of the heating element 25, serve fo increase the delay in current build-up through this element and the two serially related elements 23 and 24 when a voltage transient of the character described appears between the line conductors 20.

More generally considered, the time-current characteristics of one and two ampere fuses Iboth -characterized by the features of the invention described above, are respectively indicated at A and B in the graph shown in Fig. 3 of the drawings. In this graph, current through the fusible elements of the link, as measured in amperes, is plotted as a function of melting time in seconds, with the current being plotted on a logarithmic scale along the vertical axis of the graph and the melting time being plotted on .a logarithmic scale along the horizontal axis of lthe graph. From an examination of the characteristic curve A, for example, for a fuse link having a l ampere rating, it will be seen that this curve is a composite of the curves Ai and A2, individually and respectively representing the time-current characteris-tics of the fusible elements 24 and 23. The dash line projections of the two characteristic curves AI and A2 intersect at the point D, such that the solid line portions thereof, which extend in opposite directions away from the point D, represent the only portions of the two curves over which the two fusible elements 23 and 24 are operative to perform their protective functions. Thus, the curve AI considered in its entirety, indicates that the fusible element 24 is capable of conducting a sustained current of roughly 8 amperes for an indefinite period. Accordingly, that portion of ly 11/2 amperes, for example, the fusible element 23 will pass the overload current for not more than seconds after which it will rupture. From a consideration of the dash line portion of the curve A2, it will be noted that the fusible element 23 provides substantially no protection ,during current overload time intervals of less than l second regardless of the magnitude of the overload current. In the short time regions of the composite curve A, therefore, the fusible element 24 must take over the function of providing the required overload current protection. This it does, as will be seen from the curve AI. Thus, at currents in excess of 10 amperes, the fusing time for the fusible element 24 decreases rapidly with increasing current ow therethrough until a point E is reached at which the surge gap 32 is broken down. In this regard it will be seen that when a voltage transient is impressed across the fuse link I0 having a sufficiently steep wave front to increase the current through the link to a value in excess of a thousand amperes in a `time interval of less than approximately .0001 second, the gap 32 is ionized to by-pass the surge current around the fusible elements of the link before any portion of the link can -be heated to a melting temperature.V If the surge gap 32 is omitted from the link structure, the curve AI, representing the current-time characteristic of the fusible element 24, may be projected on a straight line away from the point D along the path indicated until it intersects the vertical ordinate representing the shortest time interval. The characteristic curve B for a fuse link having the higher `current rating of 2 amperes conforms in configuration and pattern to the curve A analyzed above, but is, of course, lower than the curve A Ibecause of the greater current rating of the fuse link which it characterizes.

Referring now more particularly to Fig. 4 of the drawings, there is illustrated a -modied arrangement of the fuse link wherein those parts which correspond to parts shown in Fig. 2 are identied lby the same reference characters. In the main, the embodiment of the invention shown in Fig. 4 of the drawing differs from that shown in Fig. 2 in that an expelling spring 3| is tensioned between the sleeve 28 and the sleeve 30 to act both as an expelling -device when the fuse is ruptured and as a connecting element for connecting the gap 32 in shunt with the series connected elements 25, 23 and 24. More specifically, the spring 3| surrounds the lower end portion of the sleeve 23 and, at its upper-end, is tensioned and seated against the inwardly and then outwardly curved lower end portion of the sleeve 30. Thus, the upper end of the spring 3| is restrained against transverse movement relative to the tube 29. At its lower end, this spring is anchored within curved fingers 28e which em'brace the lower turn of the spring and are stuckvout from the sides of the sleeve 28 at two or more points around the lower end portion thereof. In the assembly of the various parts, the fingers are pulled tightly around the lower turn of the spring 3|, so that the turns of the spring are in touching engagell ment, thus minimizing the impedance of the path in series with the surge gap 32.

From the foregoing explanation it will be understood that the disclosed link structure is exceedingly simple in arrangement, may be easily and cheaply manufactured, and yet provides positive protection against damage to an associated transformer or the like when the link is subjected to all types of overload currents of the character normally encountered in operating practice. It will also be noted that in several instances the individual elements of the structure are utilized to perform two or more functions. Thus the element 25 not only acts as a heating element to impart the desired time-current characteristic to the fusible element 23, but additionally functions as an electrical impedance element to insure breakdown or" the surge gap 32 before the fusible elements 23 and 2li can be ruptured by a transient of large magnitude and steep wave front. Again, the body 2l of refractory material not only functions to render the fusible element 23 nonresponsive to surge currents of short duration, but additionally functions to protect the heating element 25 against burnouts or damage when the link is subjected to exceedingly heavy surge currents. Further, the clement 2d not only functions as a surge current protective element, but additionally acts as a pull-out wire to break the electrical circuit through the fusible element 23 when the latter element is heated to a melting temperature. It is to be noted that in both ernbodiments of the invention, the electrodes 22 and defining the annular surge gap 32 are relatively immovable, such that the breakdown voltage thereacross is reasonably fixed and is unaffected by movement of the sleeve 23 within the bore of the tube part 29e. Moreover, in the Fig. 4 arrangement, the use of the spring 3l in the dual capacity of an expelling means and an electrical connecting means permits the immovable electrode structure to be used while at the same time permitting entirely free movement of the sleeve and fuse wire remnant out of the end f the tubes 22 and 29 when the fuse blows.

While two embodiments of the invention have been disclosed, it will be understood that various modications may be made therein without departing from the true spirit and scope of the invention as set forth in the appended claims.

I claim:

l. A fuse link comprising a metal casing, a fusible element and an impedance element disposed within said casing and connected in series, a conductor connected to said fusible element and movable relative to said casing to extract an unfused portion of said fusible element from said casing when said fusible element is ruptured, and means including a conductive element fixedly positioned relative to said casing and slidably engaging said conductor for defining with said casing a surge by-pass gap in shunt with said series connected elements, said conductor being slidable past said conductive element upon rupture of said fusible element to extract the unfused portion of said fusible element from said casing.

2. A fuse link comprising a tubular metal casing having an open end, a fusible element disposed within said casing, an insulating member projecting outwardly from the open end of said casing, a metallic element fixedly carried by the projecting portion of said insulating member and spaced from said casing, to define with said casing a surge gap having a fixed gap dimenSOn,

Cil

and means connecting said gap in shunt with said fusible element.

3. A fuse link comprising a tubular metal casing having an open end, an insulating tube fixedly positioned within said casing and including a part projecting outwardly from the open end of said casing, a metallic sleeve flxedly carried by the projecting part of said tube, said sleeve having a surface spaced from the open end of said casing to provide a surge gap between said casing and said sleeve, a fusible element and an impedance element disposed within said casing, said elements being connected in series between said casing and said sleeve, whereby said gap is connected in shunt therewith.

4. A fuse link comprising a tubular metal casing having an open end portion which is turned inwardly to provide a supporting shoulder, an

insulating tube fitting within said casing and having a stepped portion supported upon said shoulder and projecting outwardly from the open end of said casingfa fusible element and an impedance element -disposed within said tube and connected in series, one terminal of said impedance element being connected to said casing, a conductor connected to said fusible element and withdrawable from the projecting portion of said tube when said fusible element is ruptured, a metallic sleeve fixedly carried by the projecting portion of said tube and spaced from the open end portion of said casing to provide a surge gap therebetween, and means electrically connecting said sleeve to said conductor in a manner such that movement of said conductor through said tube is not interfered with.

5. A fuse link comprising a tubular metal casing having an open end portion which is turned inwardly to provide a supporting shoulder, an insulating tube fitting within said casing and having a stepped portion supported upon said shoulder and projecting outwardly from the open end of said casing, a fusible element and an impedance element disposed within said tube and connected in series, one terminal of said impedance element being connected to said casing, a conductor connected to said fusible element and withdrawable from the projecting portion of said tube when said fusible element is ruptured, a metallic sleeve fixedly carried by the projecting portion of said tube and having an end spaced from the open end portion of said casing to provide an annular surge gap therebetween, and at least one resilient metallic finger projecting from said sleeve and slidably engaging said conductor to connect said surge gap in shunt with said series connected fusible and impedance elements without interfering with movement of said conductor through said tube when said fusible element is ruptured- 6. A fuse link comprising a tubular metal casing having an open end portion which is turned inwardly to provide a supporting shoulder, an insulating tube fitting within said casing and having a stepped portion supported upon said shoulder and projecting outwardly from the open end of said casing, a fusible element and an impedance element disposed within said tube and connected in series, one terminal of said impedance element being yconnected to said casing, a conductor connected to said fusible element and withdrawable from the projecting portion of said tube when said fusible element is ruptured, a metallic sleeve xedly carried by the projecting portion of said tube and having an end spaced from the open end portion of said casing to provide an annular surge gap therebetween, and a coll spring tensioned between said sleeve and said conductor to connect said gap in shunt with said series connected impedance and fusible elements and to bias said conductor for movement outwardly from the projecting portion of said tube when said fusible element is ruptured.

7. A fuse link comprising a tubular metal casing having an open end, an insulating tube fitting within said casing and having a portion projecting outwardly from the open end of said casing, a conductor extending within the projecting portion of said tube, a fusible element disped within said casing and electrically connected between said casing and said conductor, said conductor being withdrawable from the projecting portion oi said tube when said fusible element is ruptured, a metallic element fixedly carried by the projecting portion of said tube and having a part spaced from the open end of said casing to provide a substantially annular surge gap therebetween, and a coil spring tensioned between said metallic element and said conductor to connect said gap in shunt with said fusible element and to bias said conductor for movement outwardly from the projecting portion of said tube when said fusible element is ruptured.

8. A fuse link comprising a tubular metal casing having an open end, insulating tube means fitting within said casing and extending throughout substantially the entire length of said casing, said tube means having portions of different internal diameters and an internal step intermediate the end thereof, a tubular conductor having a flange seated on said step and extending away from said step through the portion of said tube means having thelargcst internal diameter, a second conductor extending through the portion of said tube means having the smallest internal diameter and including a part projecting within said tubular conductor, a fusible body electrically and mechanically connecting said part of said conductor to said tubular conductor, a coiled conductor surrounding said tubular conductor and disposed within the space between said insulating tube means and said tubular conductor, said coiled conductor being connected in series with said fusible body and said second conductor, and a body of insulating material at least partially imbedding the turns of said coiled ccn- 'f ductor and at least partially lling the unoccupied space within that portion of said tube means having said largest internal diameter.

9. A fuse link comprising a tubular metal casing having an open end, an insulating tube tting within said casing and extending throughout substantially the entire length of said casing, said tube being provided with a first portion of large internal diameter disposed wholly within said casing, a second portion of smaller interna] diametei projecting through the open end of said casing and an internal step between said v two portions, a tubular conductor disposed within said first portion of said tube and provided with a flange seated on said step, a second conductor extending through said second portion of said tube and including a part projecting within said tubular conductor, a fusible body electrically and mechanically connecting said part of said second conductor to said tubular conductor, a coiled conductor surrounding said tubular conductor and disposed within the space between said insulating tube and said tubular conduct-or, said coiled conductor being connected between said tubular conductor and said casing, a Ibody of refractory material at least partially imbedding the turns of said coiled conductor and disposed within the unoccupied space within said first portion of said tube, and means including said casing and a conductive element fixedly carried by the projecting portion of said tube for defining a surge gap which shunts said series connected fusible body and coiled conductor.

10. A tubular metal casing having an open end, a fuse link comprising a fusible element disposed within said casing, a conductor connected to said element to extend out of the open end of said casing Aand adapted to be biased to separate unfused portions of said element when said element is ruptured, and means including a fixed electrode spaced from said casing and electrically connected to said conductor for defining with said casing a surge lay-pass gap normally shunting said element and having a fixed gap dimension before, during and after the movement of said conductor which occurs incident to rupture of said link.

11. A fuse link comprising a tubular metal casing having an open end, a fusible element and an impedance element connected in series and disposed within said casing, a conductor connected to one end of said fusible element and biased to separate unfused portions of said fusible element when said fusible element is ruptured, and means including a fixed electrode spaced from said casing and electrically connected to said conductor for defining with said casing a surge by-pass gap normally shunting said series connected elements and having a fixed gap dimension before, during and after the movement of said conductor incident to rupture of said link.

12. A fuse link comprising a tubular metal casing having an open end, a fusible element and an impedance element disposed within said casing, a conductor connected to one of said elements to extend out of the open end of said casing and adapted to be biased to separate unfused portions of said fusible element when said fusible element is ruptured, said fusible element and impedance element being connected in series between said casing and said conductor, and means including a fixed electrode located externally of said casing in spaced relationship therewith and electrically connected to said conductor for defining with said casing a surge by-pass gap normally shunting said series connected elements and having a, fixed gap dimension before` during and after the movement of said conductor which occurs incident to rupture of said fusible element.

13. A fuse link comprising a tubular metal casmg having an open end, a fusible element and an impedance element disposed within said casing, a conductor connected to one of said elements to extend out of the open end of said casing and adapted to be biased to separate unfused portions of said fusible element when said fusible element is ruptured, said fusible element and impedance element being connected in series between said casing and said conductor, an insulating tube fixeclly positioned within said casing and including a part projecting outwardly from the open end of said casing, a metallic sleeve fixedly carried by the projecting part of said tube and spaced from the open end of said casing to define with said casing a surge by-pass gap having a fixed gap dimension before, during and after the movement of said conductor which occurs incident to rupture of said fusible element, and means slidably engaging said conductor to connect said sleeve therewith, thereby to connect said gap in shunt with said series connected elements.

14. A fuse link comprising a tubular metal casing having an open end portion which is turned inwardly to provide a supporting shoulder, an yinsulating tube tting within said casing and having av stepped portion supported upon said shoulder and projecting outwardly from the open end of said casing, a conductor disposed Within the projecting portion of said tube, a fusible element and an impedance element disposed within said casing and connected in series between said casing and said conductor, a metallic sieeve nxedly carried by the projecting portion of said tube and spaced from the open end portion of said casing to provide a surge gap therebetween, said conductor being withdrawable from the projecting portion of said tube Awhen said fusible element is ruptured, and means electrically connecting said sleeve to said conductor in a manner such thatmovement of said conductor through said tube is not interfered with.

l5. A fuse link comprising a tubular metal casing having an open end and a closed end, insulating tube means disposed within said casing and extending from the closed end of said casing outwardly beyond the open end thereof, said tube means being provided With a first portion of large internal diameter, a second portion of smaller internal diameter and an internal step between said two portions, a tubular conductor disposed within said first portion of said tube means and provided with a flange seated on said step, a second conductor extending through said second portion of said tube means and including a part projecting within said tubular conductor, a fusible body electrically and mechanically connecting said part of said second conductor to said tubular conductor, a coiled heating conductor at least partially enclosing said tubular conductor and disposed Within the space between said insulating tube means and said tubular conductor, said coiled conductor being connected between said tubular conductor and said casing, a body of insulating material at least partially embedding the turns or said coiled conductor and at least partially iilling the unoccupied space within said first portion of said tube means, and means including said casing and a conductive element electrically connected to said tubular conductor for dening a surge gap which shunts said coiled conductor.

16. A fuse link comprising a tubular metal casing having an open end, a fusible element, a heating element for heating said fusible element, said elements being connected in series and disposed within said casing, a conductor connected to said fusible element and adapted to be biased to separate unfused portions of said fusible element when said fusible element is ruptured, and means including a fixed electrode spaced from said casing and electrically connected to said fusible element for defining with said casing a surge by-pass gap normally shunting said heating element and having a xed gap dimension before, during and after the movement of said conductor which occurs incident to rupture of said fusible element.

1'7. A fuse link comprising a tubular metal casing forming one terminal of said link and having an open end, an operi-ended insulating tube projecting from the open end of said casing, a terminal conductor projecting from the open end of said tube, means including a fusible element disposed Within said tube for providing a current conductive path between said casing and said conductor, a conductive sleeve carried by the projecting end of said tube and spaced from the open end of said casing to dene a Surge gap, and a resilient conductive element extending across the open end of said tube in engagement with said conductor, thereby to provide a conductive path therebetween while permitting Withdrawal of said conductor fromsaid insulating tube upon fusing of said fusible element.

18. A fuse link comprising a tubular metal casing forming one terminal of said link and having an open end, an open-ended insulating tube projecting from the open end of said casing, a terminal conductor projecting from the open end of said tube, means including a fusible element disposed within said tube for providing a current conductive path between said casing and said conductor, a conductive sleeve carried by the projecting end of said tube and spaced from the open end of said casing to define a surge gap, said sleeve being provided with an inturned portion overlying the open end of said tube, and a resilient conductive element supported by said inturned sleeve portion adjacent the open end of said insulating tube and engaging said conductor, thereby to provide a conductive path between said conductor and said sleeve While permitting Withdrawal of said conductor from said insulating tube upon fusing of said fusible element.

19. In a fuse link which includes a terminal coind'uctor withdrawable from the open end of an insulating tube in response to rupture of the fusible element of the link, the means for deiining a surge gap shunting the fusible element of the link which comprises a conductive sleeve carried by said tube to act as one electrode of said gap and provided with an inturned portion REFERENCES CITED The following references are of record lin the lle of this patent:

UNITED STATES PATENTS Number Name Date 662,466 Sachs Nov. 27, 1900 2,174,476 Pittman et al Sept. 26, 1939 2,305,394 Smith, Jr. Dec. 15, 1942 

